Recently, the electromagnetic wave of the terahertz band (THz band), which is the boundary zone between light and radio waves, increasingly attracts interests. In many cases, the terahertz electromagnetic wave generally means the electromagnetic wave in a frequency domain of 100 GHz-10 THz, and the wavelength thereof is approximately not more than 300 μm. It is expected that the above terahertz electromagnetic wave be applied to nondestructive inspection, imaging, communication or the like. The application of terahertz electromagnetic wave has expanded to a field of environmental measurement and life science; thus, the terahertz electromagnetic wave has emerged as a field of leading basic technology.
One of the devices that generate and detect such terahertz electromagnetic wave is an optical switch element (terahertz electromagnetic-wave device). That is, the optical switch element is capable of generating and detecting the terahertz electromagnetic wave by being pumped by a femtosecond laser pulse. Therefore, the optical switch element is required to operate at an ultra high speed so as to respond to the femtosecond laser pulse. In addition to the above, the semiconductor material used in the substrate portion of the optical switch element, which generates carriers by means of pumping of the laser pulse, is required such characteristics as high resistance, high mobility, short carrier life time and the like. In these required characteristics, the characteristic of the carrier life time affects largely on the detection area and the SN-ratio when the optical switch element is applied to detect the terahertz electromagnetic wave. Therefore, the characteristic of the carrier life time is particularly important.
As a method of obtaining a semiconductor substrate with a short carrier life time to be applied to the above-mentioned optical switch element, a conventional method, in which crystal defects are introduced into the substrate by means of ion implantation, is known. According to this method, the crystal defects function as an essential element that captures the carriers. And thus, a semiconductor substrate, of which carrier life time is approximately 1 ps, can be obtained. However, in the method, in which the carrier life time is made to be short by means of the ion implantation, there resides such problem that the mobility, which is one of the required characteristics for the optical switch element, is greatly reduced.
Therefore, in many cases, a semiconductor substrate (LT-GaAs substrate), in which a GaAs epitaxial layer is grown on a semi-insulating substrate comprised of GaAs or the like at a substrate temperature lower than ordinal temperature by means of molecular beam epitaxy (MBE), is used. In this substrate, As is excessively implanted while the epitaxial layer is allowed to grow at a low temperature; and after that, the As is deposited as As-clusters by a required heat treatment. The As-clusters function as a main factor that captures the carriers. Owing to this, in this substrate, it is possible to obtain a short carrier life time and a relatively high mobility without causing a large damage on the crystal.
For example, in Document 1: S. Gupta, IEEE, J. Q. Elec., 28 (1992), pp. 2464, the above-mentioned LT-GaAs substrate is disclosed. By using this substrate, the carrier life time shorter than 1 ps and the mobility of approximately 100 to 200 cm2/Vs have been achieved. Also, in Document 2: the Japanese Laid-Open Patent Publication No. 2002-257629, a technique to generate terahertz electromagnetic wave and a technique to detect the same by using an optical switch element, in which a dipole antenna is formed on the LT-GaAs substrate with the life of the carriers reduced. Further, in Document 3: the Japanese Laid-Open Patent Publication No. 07-36064, a technique relevant to the reduction of the life of the carriers in an InGaAs compound semiconductor to be used in a 1.3 to 1.55 μm band, which is essential in the field of communication. By doping Be to a substrate, the carrier life time is reduced.